Strangler Fig Pattern

To incrementally migrate a legacy system, you identify seams with clear input and output boundaries, introduce an abstraction layer to route traffic, and gradually replace old components while preserving production stability.

The strangler fig pattern is a legacy migration technique that gradually replaces old system components around a host. You should use it for complex migrations involving production traffic, phased rollouts, and continuous delivery.

To safely roll out a new service during a legacy migration, implement the new version behind feature flags and execute a phased rollout using dark launch, canary deployment, and ramp-up stages with active monitoring.

To identify seams for a legacy migration, locate clear input and output boundaries in the legacy code, then create an abstraction layer to route traffic between old and new implementations while adding characterization tests.

You can maintain production stability during component replacement by enforcing an abstraction layer, instrumenting safety checks, defining rollback criteria, and executing staged deployments across APIs, services, and data boundaries.

You should avoid a phased rollout for replacing legacy systems when you cannot establish characterization tests, enforce an abstraction layer, or define clear rollback criteria to ensure production stability during the migration.